A logarithmic amplifier (“log amp”) generates an output signal VOUT that is related to its input signal VIN by the following transfer function:VOUT=VY log(VIN/VZ)  Eq. 1where VY is the slope and VZ is the intercept. To provide accurate operation, VY and VZ should be stable over the entire operating temperature range of the log amp. In a monolithic implementation of a progressive compression type log amp, temperature compensation of the slope VY is typically provided in the gain and detector cells since those are the structures that determine the slope. Temperature stabilization of the intercept VZ, however, is typically provided at the front or back end of the log amp. For example, a passive attenuator with a loss that is proportional to absolute temperature (PTAT) may be interposed between the signal source and the log amp. Such an arrangement is disclosed in U.S. Pat. No. 4,990,803.
Another technique for temperature compensating the intercept of a log amp involves adding a carefully generated compensation signal to the output so as to cancel the inherent temperature dependency of the intercept. The intercept VZ of a typical progressive compression log amp is PTAT and can be expressed as a function of temperature T as follows:
                              V          Z                =                              V                          Z              ⁢                                                          ⁢              0                                ⁡                      (                          T                              T                0                                      )                                              Eq        .                                  ⁢        2            where T0 is a reference temperature (usually 300° K.) and VZ0 is the value of VZ at T0. Substituting Eq. 2 into Eq. 1 provides the following expression:
                              V          OUT                =                              V            Y                    ⁢                      log            ⁢                                                  [                                          (                                                      V                    IN                                                        V                                          Z                      ⁢                                                                                          ⁢                      0                                                                      )                            ⁢                              (                                                      T                    0                                    T                                )                                      ]                                              Eq        .                                  ⁢        3            which can be rearranged as follows:
                              V          OUT                =                                            V              Y                        ⁢            log            ⁢                                                  ⁢                          (                                                V                  IN                                                  V                                      Z                    ⁢                                                                                  ⁢                    0                                                              )                                -                                                                      V                  Y                                ⁢                log                ⁢                                                                  ⁢                                  (                                      T                                          T                      0                                                        )                                            ︸                                      Temperature              -              dependent                                                          Eq        .                                  ⁢        4            It has been shown that accurate intercept stabilization can be achieved by adding a correction signal equal to the second, temperature-dependent term in Eq. 4 to the output of a log amp, thereby canceling the temperature dependency. See, e.g., U.S. Pat. No. 4,990,803; and Barrie Gilbert, Monolithic Logarithmic Amplifiers, August 1994, § 5.2.4. A prior art circuit for introducing such a correction signal is described with reference to FIG. 19 in U.S. Pat. No. 4,990,803.